Assignment 5 – C program Unix environment – processing a sequence of commands System Programming

The C program below, behaves like a small shell. It processes
single commands entered by the user. In particular, the program assembles commands and execute them, either in the background or foreground. The commands/programs location can be anywhere in $PATH and might have arguments.
Modify this solution to make the program capable of processing a sequence of commands/
programs separated by ’;’.
E.g., if the input-line entered by the user is
ls -F; pwd; google-chrome&; date
your program should assemble these 4 commands and execute them in sequence.
The example to be modified:
Algorithm:
While(1)
begin
get command line from user
assemble command args
duplicate current process
child should exec to the new program
if (background execution) then
parent process accept without waiting for its child to terminate
else
parent process waits for its child to terminate
end

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
int readArgs(char *, char *[]);
int main(int argc, char *argv[]){
pid_t pid;
char line[255], *argList[20];
int ampersand, status, i;

printf("This program will execute commands/programs for you\n");
while(1){
printf("To exit, enter CTR-C, or\n");
printf("enter a program name with its arguments> ");
fgets(line, 255, stdin);
ampersand=readArgs(line, argList);
if((pid = fork()) == -1){
perror("impossible to fork");
exit(1);
}
if(pid > 0) // This is parent
if(ampersand){
waitpid(pid, &status, WNOHANG);
printf("Process [%d]\n", pid);
}
else{
waitpid(pid, &status, 0);
printf("My child has terminated\n");
}
else // this is the child
if(execvp(argList[0], argList)==-1){
perror("child Process");
exit(22);
}
}
exit(0);
}
int readArgs(char *line, char *argList[]){
static int yes=0;
int i=0, offset=0, found=0;
char name[50];
while(yes & argList[i] != NULL)
free(argList[i++]);
i=0;
while(sscanf(line+offset, "%s", name)==1){
argList[i] = (char *) malloc(strlen(name)+1);
strcpy(argList[i++], name);
while(line[offset] == ’ ’) // skip extra blanks
offset++;

offset += strlen(name);
}
if(!strcmp(argList[i-1], "&")){
argList[i-1] = NULL;
found = 1;
}else{
if(argList[i-1][strlen(argList[i-1])-1] == ’&’){
argList[i-1][strlen(argList[i-1])-1]=’\0’;
found = 1;
}
argList[i] = NULL;
}
yes=1;
return(found);
}

Answer:

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

int readArgs(char *, char *[]);
int separate(char *, char *[]);
pid_t myFork();

int main(int argc, char *argv[])
{
    pid_t pid;
    char line[255];
    char *commandList[10];
    int nbCommands;
    char *argList[20];
    int ampersand, status, i;
    
    printf("This program will execute commands/programs for you\n");
    
    for(i=0; i<10; i++)
        commandList[i] = (char *)malloc(100);
    
    while(1)
    {
        printf("To exit, enter CTR-C, or\n");
        printf("enter a command List separated by ;> ");
        fgets(line, 255, stdin);
        
        nbCommands = separate(line, commandList);
        
        for(i=0; i < nbCommands; i++)
        {
            strcpy(line, commandList[i]);
            printf("\n Executing %s\n", line);
            sleep(1);
            
            ampersand=readArgs(line, argList);
            
            if((pid = myFork()) == -1)
            {
                perror("impossible to fork");
                exit(1);
            }
            
            if(pid > 0)  
                if(ampersand)
                    printf("Process [%d]\n", pid);
                else
                {
                    waitpid(pid, &status, 0);
                    printf("My child has terminated\n");
                }
                else                            
                    if(execvp(argList[0], argList)==-1){
                        perror("child Process ");
                        exit(22);
                    }
        }
    }
    
    exit(0);
}



int readArgs(char *line, char *argList[])
{
    static int yes=0;
    int i=0, offset=0;
    char name[50];
    int found=0;
    
    while(yes && argList[i] != NULL)
        free(argList[i++]);
    
    i=0;
    
    while(sscanf(line+offset, "%s", name)==1)
    {
        argList[i] = (char *) malloc(strlen(name)+1);
        strcpy(argList[i++], name);
        while(line[offset] == ' ')
            offset++;
        offset += strlen(name);
    }
    
    if(!strcmp(argList[i-1], "&"))
    {
        argList[i-1] = NULL;
        found = 1;
    }
    else
        {
        if(argList[i-1][strlen(argList[i-1])-1] == '&'){
            argList[i-1][strlen(argList[i-1])-1]='\0';
            found = 1;
        }
        argList[i] = NULL;
    }
    
    yes=1;
    return(found);
}



pid_t myFork()
{
    static int count=0;
    
    count++;
    if(count <= 20)
        return(fork());
    else
        return(-1);
}



int separate(char *line, char *commandList[])
{
    int i=0, j=0, k=0;
    
    while(line[i] != '\n'){
        if(line[i] != ';'){
            commandList[j][k] = line[i];
            k++;
        }
        else
            {
                
            commandList[j][k] = '\n';
            commandList[j][k+1] = '\0';
            k=0;
            j++;
            i++;
                
            }
        
        i++;
    }
    
    commandList[j][k] = '\n';
    commandList[j][k+1] = '\0';
    
    return(j+1);
}